The present invention relates to optical fibers and, more particularly, to optical fiber sensors that measure physical parameters such as magnetic field, temperature, pressure, and the like.
Optical fibers are well known as conduits of optical energy whose primary use has been for the communication of information. A typical optical fiber of the simplest kind is a long filament comprising an inner transparent rod or core which may be glass or plastic, of a relatively high refractive index. The core is conventionally surrounded by a second region of relatively low refractive index, called the cladding. By virtue of the optical principle of mode propagation, light is conveyed along the length of this structure with relatively low loss of optical energy.
The term "core" refers generally to any guiding region of relatively high refractive index which may be located within the structure of the fiber. It need not be a cylindrical rod shaped region only. Rods of non-circular cross-section or ring shaped guiding regions are also known in the art. The essential electromagnetic properties with respect to the guiding of optical energy of all cross-section geometries are similar.
Optical fibers are also known which have structures encompassing two cores of relatively higher refractive index embedded within a single cladding of relatively lower index. Such fiber configurations are typically used for the purpose of dual channel communications or secure communications or as coupling or wavelength filtering devices. Dual core fibers may be in the form of two rod-type cores, or a rod core surrounded by a ring shaped second core, or two concentric ring cores, or any other combination of like or unlike cores of any geometrical cross-section incorporated within the same cladding structure.
While communications is the major application of optical fiber technology, interest has developed recently in another area: the use of the physical and optical properties of the fibers themselves to construct sensors, instruments or measurement devices. These sensors respond to conditions such as temperature, pressure, strain, fluid flow, magnetic or electric fields, or other parameters.
A number of different modes of the electromagnetic field propagated in the fiber can be controlled as a function of the diameter of the core, the wavelength of the light launched into it, and the values of the refractive indices of the core and cladding materials. It is possible to arrange these parameters so that only one mode is supported. Such single-mode fibers are useful for communications purposes, or more generally when it is desirable to utilize the interference properties of the electromagnetic fields in the fiber.
Heretofore, the most sensitive fiber optic sensing devices have been assemblages of single-mode optical fibers combined in some fashion to manifest phenomena of wave interference between the optical fields conveyed by two or more fibers. A laser source is generally divided into two parts initially and launched into two separated fibers.
U.S. Pat. No. 4,173,412 issued to Ramsay et al. discloses a sensor for measuring strain. The sensor operates on the principle that transverse straining of a single-mode fiber produces birefringence effects which can be observable using polarized light.
U.S. Pat. No. 4,295,738 issued to Meltz et al. discloses an optical fiber having two cores and being sensitive to strain or hydrostatic pressure through choice of materials, spacing and shape of the cores and cladding of the fiber. The strain or pressure change is measured by the relative intensity of light emerging from each core as a result of cross-talk between adjacent cores.
U.S. Pat. No. 4,295,739 issued to Meltz et al. discloses a multi core optical fiber having a plurality of cores in a common cladding which respond to either temperature or strain through choice of materials, spacing and shape of the cores of the fiber. The temperature strain or pressure change is measured by the relative intensity of light emerging from each core as a result of cross-talk between adjacent cores.
U.S. Pat. No. 4,151,747 issued to Gottlieb et al. discloses an arrangement for monitoring temperature. An optical fiber cooperates with a light source and a detector to sense changes in the temperature by monitoring the amount of light which passes through the fiber.
The foregoing references all disclose devices in which one fiber is subjected to influences by means of which external environmental fields alter the phase of the optical field within the fiber.
Also, it is known that certain coating materials applied to the outside of single-mode optical fibers can transduce environmental conditions such as pressure or magnetic or electric fields into strains in such fibers, thereby modulating the optical phase and resulting in fiber optic sensing of said conditions. As another example, temperature sensitivity can be enhanced by coating fibers with metallic films.
Optical interferometry has not been widely applied to manufactured devices because of a need for complex and highly precise mechanisms that are stable to within a fraction of one light wavelength. Single-mode optical fibers, as exemplified by devices such as interferometric fiber optic sensors, offer a means to package and miniaturize the techniques of interference of light waves.
Interferometric fiber optic sensors constructed from combinations of two or more separate optical fibers are subject to disadvantages. They are susceptible to mechanical vibration and thermal gradients among their several parts.
An improved class of devices can be designed by incorporating the two elements of a fiber interferometric device in the form of a dual core optical fiber. If two parallel cores of equal propagation constants are placed sufficiently close together, the electromagnetic fields propagating in each overlap in the cladding region, causing an interaction between cores known as evanescent wave coupling.
The aforementioned U.S. Pat. Nos. 4,295,738 and 4,295,739 issued to Meltz et al. and U.S. Pat. No. 4,151,747 issued to Gottlieb et al. disclose devices which take advantage of the process of evanescent wave coupling between the cores of a dual core fiber to manifest an optical fiber sensor of temperature or strain within the structure of a single fiber. A dual core evanescent wave device is a differential interferometer. Such devices are simpler and inherently more stable than fiber interferometers involving two fibers. Optical power is launched into one of the two cores. Evanescent wave coupling along the entire length of the dual core structure acts to produce a transfer of light to the second core and then back to the first, in a periodic manner characterized by a beat length which reflects the strength of the interaction. Since beat length is sensitive to temperature and strain of the fiber, it may be used to sense or measure these parameters. In the referenced prior art, the dual or multiple cores all have identical propagation constants. Such cores are said to be phase matched, allowing light energy to couple between the cores.
Dual core evanescent wave fiber optic sensors with phase matched cores are subject to a number of limitations. First, only temperature and strain can be measured. Second, the evanescent wave coupling action and hence the sensitivity of the dual core structure acts along the entire length of the fiber. The conveyance of optical power to and from the sensor is accomplished by joining the structure to a number of conventional single core fibers acting as leads. These connections are usually accomplished by means of various connectors and joints which may complicate the use of the device and decrease efficiency thereof.
It would be advantageous to provide a sensitive fiber in which the type and location of sensitivity to external influences could be controlled and restricted to portions of the length so that separate leads were not required.
It would still further be advantageous to provide an optical fiber in which a plurality of locally sensitized areas can be disposed along the same fiber, each area being sensitive to different environmental effects.